Photosensitive resin compositions can be used for producing printing plates. Recently it has been recently desired for health and safety reasons to use photosensitive resin compositions which are water developable, i.e. which are developed with water or an aqueous medium after exposing to light.
Applicants proposed in Japanese Kokai Publication 238598/1987 that radically polymerized crosslinked resin particles, which are prepared by an emulsion polymerization method, be formulated into a photosensitive resin composition. According to the proposed technique, the water dispersibility of the composition is enhanced and, therefore, the water developability of a resin plate is enhanced. Also, the rusulting printing plate has excellent water resistance which facilitates the use of an aqueous printing ink therewith. It is, however, desired that the proposed composition is further improved in water developability and storage stability to satisfy the requirement of users.
Crosslinked resin particles can be prepared by many methods, such as by an emulsion polymerization method, a grinding method and a so-called post-emulsion method (Japanese Kokai Publication 156717/1985). In the post-emulsion method, a resin is dispersed in water and then dimensionally crosslinked within the dispersed resin particles through reactive groups present in the resin to form crosslinked resin particles which are then removed from the water.
In the post-emulsion method, since the surface of the crosslinked resin particles is easily plastified, fusion bonds frequently form between particles during the water removing step, resulting in large fused particles. In such instances, undesired water is trapped in the large fused particles. These problems are more pronounced when the resin component to be dispersed in water is thermoplastic and has a low glass transition temperature (Tg), such as for example, acryl rubber, polybutadiene, polyisoprene, polychloroprene, poly-epsilon-caprolactone and polytetramethyleneglycol. Crosslinked resin particles prepared by the post-emulsion method from such thermoplastic resins have poor redispersibility in an aqueous medium, consequently these resin paricles have not been practically produced.